ON THE FACTS AND THEORY OF EARTHQUAKE PHENOMENA. 83 
in wetness or dryness, in temperature, and in many other ways. Stratifica- 
tion and lamination, with their transverse master-joints, affect the elasticity 
of whole mountain-ranges and profound masses of the land, and cause it to 
differ in different directions. 
The mass beneath our feet is very often not even approximately solid. 
Vast beds and cavernous recesses occur, empty, or filled more or less with 
water, sometimes with lava, ignited rock, and steam at enormous temperature 
and tension; and, for anything we as yet know, scismometry may require to 
deal with depths and masses where the solid has passed, with exalted tem- 
perature, into the imperfectly liquid state. 
Again, the surface of our earth is everywhere more or less uneven, and, 
viewed over large areas, such as earthquake-transit is concerned with, is 
ribbed with rigid mountain-chains, often intersecting or abutting on each 
other, channeled by valleys, river-courses, deep estuaries, and bays, exca- 
vated into basin-shaped hollows often long and narrow, sometimes filled with 
unconformable rock or with loose and incoherent detrital material, and inter- 
sected to unknown depths by dykes, veins, and faults. The result of these 
differences and disturbances of internal structure and superficial features is to 
produce perturbations in the surface emergence of the earth wave, often of the 
most amazing and perplexing character ; and it is not until the nature and 
extent of these have been realized to the mind, that we shall be enabled to 
choose the best form of seismometric observation, to determine upon the only 
proper sites for the establishment of instruments, and to see within what 
limits our first researches must be confined. 
Let us notice, then, a few examples of striking surface-perturbation, of 
direction, of the great earth-wave, already on record. 
Savi (‘ Relazione di Fenomeni presentati dai Terremoti di Toscana, dell’ 
Agosto 1846,’ p. 32-44) and Pilla (‘Istoria del Tremuoto che ha devastato 
paesi della Costa Toscana il di 14 Agosto, 1846,’ p. 48-54) have both recorded 
examples of horizontal apparent movement of the earth-wave in directions 
orthogonal or even actually opposite to each other, and at points within very 
limited distances from each other, while, on the whole, there was no doubt 
of a ruling general direction of horizontal movement over the whole region. 
I can merely refer to their relations, as scarcely admitting of condensation 
intelligibly. 
M. Perrey, in his ‘Memoir on the Earthquakes of France, Belgium, and 
Holland’ (Mém. Cour. de l’Acad. Roy. de Brux. tom. xviii.), under date of 
5th July, 1841, has recorded a still more remarkable instance of surface- 
perturbation, which the small map (Plate XII.) of the northern and part of 
the central region of France, with outlines of the departmental divisions, illus- 
trates. Those departments in which this shock was felt are marked by numerals 
referring to the following table. The directions of the horizontal component 
of the shock, as observed at the several places named, are shown on the map 
by ashort thick arrow. A few other places where the shock was felt, but 
direction not observed, are marked by a large dot, and the name referred to 
by a letter. A few large towns, and the general range of the hilly country 
(running mainly in a N.W.and S.E. direction) between the two great seats of 
disturbance, are marked in mainly as general guides of position to the eye. 
This earthquake was sufficiently powerful to disturb furniture, move objects 
visibly, and affect clocks, &c., and was variously reported to have lasted in 
different places from two or three, to ninety seconds of time. 
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